1. Field of the Invention
The present invention relates to an optical film and a method for manufacturing the same, a polarization plate, and a liquid crystal display apparatus.
2. Description of the Related Art
In recent years, a liquid crystal display apparatus has been widely used as liquid crystal panels and the like in liquid crystal televisions, personal computers, mobile phones, digital cameras, and the like. Generally, a liquid crystal display apparatus includes a liquid crystal panel member provided with polarization plates on both sides of a liquid crystal cell, and is made to display images by controlling light from a back light member using the liquid crystal panel member. Here, the polarization plate is made up of a polarizer and protective films on both sides of the polarizer. An ordinary polarizer is obtained by dyeing a stretched polyvinyl alcohol (PVA)-based film using iodine or a dichromatic colorant, and a cellulose ester film or the like is used as the protective film.
In addition, it is known that, in the liquid crystal display apparatus, a polymer film is used as an optical compensation film (phase difference film) to enlarge a viewing angle, improve image coloration, and improve contrast. For the polymer film used as the optical compensation film, there has been a demand to have desired optical anisotropy by controlling the optical characteristics (for example, birefringence such as the retardation value Re in a film plane or the retardation value Rth in the film thickness direction) of the film depending on the mode of the liquid crystal cell in the liquid crystal display apparatus such as a VA mode or an IPS mode.
In response to diversification of usage as well as improvement in quality, there has been a strong demand for recent liquid crystal display apparatuses to have durability. Examples of the demanded durability include stability of the liquid crystal display apparatus against environmental changes in outdoor use and suppression of changes in dimensions or optical characteristics against changes in temperature or humidity in optical films such as the protective films for the polarization plates or the optical compensation films used in the liquid crystal display apparatus. When exposed to high temperature and humidity environments, the liquid crystal display apparatus has problems with warpage of the liquid crystal cell and the occurrence of display unevenness in the liquid crystal display apparatus, and these problems are considered to result from the fact that, when moisture permeates and is desorbed to the polarization plates and the optical films that constitute the polarization plates, the shrinkage balance between the polarization plates on the front and back surfaces of the liquid crystal cell in the liquid crystal display apparatus breaks, the liquid crystal cell warps, and the four corners or four sides of the liquid crystal cell come into contact with a housing or a member on the back surface side, thereby causing display unevenness. Therefore, there has been a demand for improvement of the humidity dependency or heat and humidity durability of the protective films for the polarization plates, the optical compensation film, and the like, and, to achieve fundamental improvements, it is necessary to suppress the permeation and desorption of moisture from environmental changes, and particularly, the optical film on the outermost surface of the polarization plate is required to have a level of performance not easily allowing the permeation of moisture, that is, the reduction of moisture permeability.
WO2009/047924A discloses an optical film obtained by adding a large amount of an acryl resin such as polymethyl methacrylate (PMMA) to cellulose ester for the purpose of providing an optical film having high transparency, low moisture absorbency, strong heat resistance, and high mechanical strength.